A conventional commercial aircraft has a fuselage (tube) and wing configuration, and a propulsion system that provides thrust. The propulsion system generally includes two or more jet engines such as turbofans. The jet engines may be mounted to the aircraft in a variety of ways. For example, the jet engines may be suspended beneath the wing, blended with the wing or mounted directly to the fuselage. The jet engines are typically installed at a distance from the fuselage and/or the wing, such that the jet engines and the fuselage interact with separate freestream airflows, thus reducing turbulence of air entering an inlet portion of the jet engine. The net propulsive thrust of the jet engines is directly proportional to the difference between jet engine exhaust velocity and freestream velocity of the air approaching the engine while in motion.
Drag, such as skin friction, form and induced drag have a direct effect on net propulsive thrust of the propulsion system. Total aircraft drag is generally proportional to a difference between freestream velocity of air approaching the aircraft and an average velocity of a wake downstream from the aircraft and that is produced due to the drag on the aircraft. Various parameters of the jet engine such as jet engine diameter, thrust capability, fan pressure ratio (FPR) for a turbofan jet engine and/or jet engine exhaust velocity must be sized and/or designed to accommodate for the total aircraft drag.
Systems and/or technics have been proposed to counter the effects of drag and/or to improve efficiency of the jet engine. For example, various propulsion systems incorporate boundary layer ingestion systems such as one or more boundary layer ingestion fan(s) and/or related techniques that route a portion of relatively slow moving air which forms a boundary layer across the fuselage into the jet engine at or upstream from a fan section of the jet engine. While this technique reduces the net drag by re-energizing the boundary layer downstream from the aircraft, the flow of air from the boundary layer entering the jet engine generally has a non-uniform or distorted velocity profile. As a result, conventional turbofan jet engines, particularly those turbofans mounted under-wing, may experience loss of operability or efficiency, thus minimizing or negating the benefits of reduced drag on the aircraft.
Accordingly, a gas-electric propulsion system that reduces net drag on the aircraft while increasing overall propulsion system efficiency and/or that allows for reduced engine diameter and/or fan pressure ratio for wing-mounted turbofans would be welcomed in the technology.